Tippint Points in the Tundra

The recent news from the Arctic is troubling. A new report (1) from NASA and the National Snow and Ice Data Center (NSIDC) indicates that the extent of sea ice cover in the Arctic Ocean is now at its lowest level in more than a century. The NASA-NSIDC team has observed four straight years of substantially below-average sea ice, with earlier spring melting and sharp declines in winter ice cover. This comes on the heels of another report by Overpeck et al. (2), supported by the NSF Arctic System Science program, which suggests that the Arctic is heading toward a new, seasonally ice-free state--a condition not seen for at least a million years. The authors are blunt: "The Arctic system is moving toward a new state that falls outside the envelope&of recent Earth history. This future Arctic is likely to have dramatically less permanent ice than exists at present&a summer ice-free Arctic Ocean within a century is a real possibility&." Overpeck et al. conclude that "The change appears to be driven largely by feedback-enhanced global warming, and there seem to be few, if any, processes or feedbacks within the Arctic system that are capable of altering the trajectory&."

Now, turning to the continents surrounding the Arctic Ocean, Chapin et al. report new findings on page 657 of this issue (3) that confirm that substantial warming over the landmasses of the Arctic is also happening, and is accelerating. In fact, from the 1960s to the 1980s, the Arctic landscapes warmed by roughly 0.15°C per decade, and then the region warmed by nearly 0.3° to 0.4°C per decade since the 1990s.

According to Chapin et al., the accelerated warming over the high-latitude continents appears to be the result of strong positive feedbacks from the land surface on a warming atmosphere. In particular, they suggest that greenhouse warming is now reducing the duration of seasonal snow cover in the Arctic, shortening the snowcovered season by roughly 2.5 days per decade, thereby shifting the albedo (the reflectivity of the surface to sunlight) of the landscape away from bright snow toward darker vegetation and soil. This decrease in albedo allows the ground to absorb more solar radiation, warm the surface, and then provide additional heat to the atmosphere (see the figure, top and middle panels). Chapin et al. estimate that this reduction in snow cover, and associated decrease in albedo, resulting from global warming adds another ~3 W m-2 of local heating to the atmosphere--an amount that is roughly comparable to what a doubling of CO2 levels would do the global atmosphere.

But changes in snow cover may not be the only feedback process at work in Arctic landscapes. Global warming may also encourage more shrubs to grow in the tundra, and boreal forest to grow farther northward, replacing the tundra ecosystems that exist there today. These changes in the land surface (to a landscape with more shrubs and trees) also profoundly affect the heat transfer between the surface and the atmosphere (see the figure, bottom panel). Although the extent of vegetation expansion in the Arctic has been relatively small so far, it is likely to continue in response to global warming and be a major factor in shaping the climate of the region. From their observations, Chapin et al. conclude that widespread shrub and tree expansion could further magnify atmospheric heating over Arctic landmasses by another factor of 2 to 7.

But how well do these new observations fit with the predictions of global climate models (GCMs)? The positive feedbacks on global warming stemming from the reduction in snow cover are already included within GCMs. Nearly all of the models include representations of the physics of land-surface processes, including the energy, moisture, and momentum balance among vegetation, soil, snow, and the atmosphere. As a result, GCMs show strong warming in the polar region; in these areas, the simulated warming is amplified through albedo feedbacks from reduced snow and ice.

Unfortunately, few GCMs represent the possible feedbacks from changing vegetation cover and the associated changes in land-surface properties. As Chapin et al. suggest, increases in shrub and forest cover in the Arctic could dramatically amplify global warming in the Arctic, but nearly all GCMs used today do not consider such changes in vegetation cover. However, a study by Levis et al. (4) used one of the few fully coupled global climate-vegetation models to estimate the potential for vegetation feedbacks on Arctic climate. They concluded that the northward shift of trees and shrubs induced by global warming would raise seasonal temperatures by an additional 1.1° to 1.6°C in spring. Naturally, further investigations with alternative models of climate- vegetation interactions are needed to corroborate this kind of result. But Chapin et al. have now provided us with strong empirical evidence to support this hypothesis.

In a way, the Arctic may be the "canary in the coal mine" of our global climate system. Climate theory and models have both suggested that the Arctic region will experience some of the strongest effects of global warming, mainly because of the large magnifying effects of snow, ice, and (possibly) vegetation feedbacks. And now several sources of evidence are showing that not only is the Arctic warming, but also that the feedback mechanisms seem to be kicking into high gear.

Ultimately, this research leads one to wonder whether the Arctic is headed toward a fundamentally different climatic regime--one with much less snow, much less sea ice, and possibly more shrubs and forest. Furthermore, scientists and decision- makers must ask what this radically different climate future means for the species and peoples that call the Arctic home today, including polar bears, seals, and Inuit communities. And given the massive inertia of the global climate system-- with the significant degree of additional warming already "in the pipeline," even if CO2 levels were to stabilize today (5)--combined with the difficulty of achieving drastic decreases in greenhouse emissions anytime in the near future, one also has to ask: Is the Arctic we know today already lost? To answer these questions, studies like that of Chapin et al. demand more attention.

The Arctic system is moving toward a new state that falls outside the envelope of glacial-interglacial fluctuations that prevailed during recent Earth history. This future Arctic is likely to have dramatically less permanent ice than exists at present. At the present rate of change, a summer ice-free Arctic Ocean within a century is a real possibility, a state not witnessed for at least a million years. The change appears to be driven largely by feedback-enhanced global climate warming, and there seem to be few, if any, processes or feedbacks within the Arctic system that are capable of altering the trajectory toward this ``super interglacial' state. For nearly 30 years, Arctic sea ice extent [e.g., Stroeve et al., 2005] and thickness [ Rothrock et al., 2003] have been falling dramatically (Figure 1). Permafrost temperatures are rising and coverage is decreasing [Osterkamp and Romanovsky, 1999]. Mountain glaciers and the Greenland ice sheet are shrinking [Meier et al., 2003; Box et al., 2004]. Evidence suggests we are witnessing the early stage of an anthropogenically induced global warming superimposed on natural cycles [Intergovernmental Panel on Climate Change, 2001], reinforced by reductions in Arctic ice.